RSYST is a modular system for reactor and shielding calculations. It consists of a system nucleus, utility modules, general data manipulation modules, reactor statics, burnup and shielding calculation modules, a data base and fast and thermal cross section libraries based on ENDF/B-IV. The computational path is defined in the card input using user control commands. User commands feature logical branches which permit the construction of loops in the module execution. Standard computational paths can be stored away in the data base and can later be activated by standard commands. RSYST stores all information in so-called data blocks of the data base. Each block in addition to the actual data contains data structure information. The system is integrated and data can be transferred between computational modules without need for rearrangement or transformation. The system nucleus takes care of the transfer of data between the data base and the modules, the utility modules can be used to prepare data for use by subsequent modules (normalizations, interpolations, comparison of data, transformations through general mathematical functions), for editing and plotting data sheets.

The computational modules are able to solve several reactor physics problems by the following methods:

- Zero-dimensional spectrum calculations in the fast energy range by the Bn and/or Pl method.
- Zero-dimensional spectrum calculations in the thermal energy range by the Bn method.
- Calculation of fluxes in the resonance domain by the Nordheim or NR approximation.
- Calculation of group constants by the gas model.
- Homogenization and collapsing of group constants.
- One-dimensional and two-dimensional neutron diffusion.
- One-dimensional transport problems by the Sn method.
- One-dimensional transport by the collision probability method.
- Burnup equations with given flux conditions.

No dynamic storage allocation is used. Some modules have fixed dimensions, some a unique data array, whose length is defined at compilation time and depends on the available storage in the computer used.

800 K bytes of fast core is the size necessary if the single modules are run without overlay. For small applications this size can be reduced considerably. Disc drives are necessary for random access files.